This invention relates generally to fiber optics and more specifically to an apparatus for reducing the polarization mode dispersion in a fiber optic transmission system.
Polarization mode dispersion (PMD) refers to variations in the time delay of a polarized optical signal traveling through an optical transmission system, such as a single-mode optical fiber. PMD arises in an optical fiber as a result of asymmetries in the optical fiber""s core, such as core ellipticity created during optical manufacturing and bending stresses formed during installation. The asymmetries of the fiber""s core cause random changes in the state of polarization (SOP) of optical signals propagating through the fiber. Different SOPs propagate through the optical fiber core at different relative speeds, e.g., some SOPs travel faster and some travel slower, resulting in a pulse width distortion of a transmitted optical signal relative to an input optical signal. Additionally, the asymmetries of the fiber""s core are highly susceptible to environmental fluctuations, such as temperature or movement of the fiber, which occur as fast milliseconds and result in a time varying pulse width distortion of the transmitted optical signal.
The varying pulse width distortion is mathematically represented by a time delay between two orthogonally polarized principal states of polarization (xe2x80x9cPSPxe2x80x9d) which form a convenient basis set to describe and characterize each SOP, and evaluate the effects of PMD in the fiber. Using the PSPs as a basis set, each SOP propagating through an optical fiber is represented by a linear combination of the two orthogonally polarized PSPs. The varying pulse width distortion of the SOP is a function of a varying delay between the PSPs. Theoretically, each PSP experiences a time of flight difference through the optical fiber, commonly known as differential group delay, resulting in a time delay between the two PSPs at the fiber output. The output SOP is represented by a linear combination of the PSPs which are time delayed with respect to each other. A greater time delay between the PSPs corresponds to a larger relative difference between the input SOP pulse width and the output SOP pulse width. See for example C. D. Poole and R. E. Wagner, xe2x80x9cA Phenomenological Approach to Polarization Dispersion in Long Single-Mode Fibers.xe2x80x9d Electronic Letters, Vol. 22, pp. 1029-1030, September 1986, which is incorporated by reference herein.
Optical fibers have a differential group delay (DGD) between the two PSPs on the order of 0.1 ps/km. In older fiber optic cables, such as the cables used in terrestrial networks, the DGD is on the order of 2.0 ps/km and results in time delays of about 50 picoseconds for transmission distances of only several hundred kilometers. As the demand for faster optical data transmission increases, such as from gigabits per second to terabits per second, optical pulse width distortion due to PMD will become one of the factors limiting data transmission rate.
A real-time optical compensating apparatus reduces first-order PMD in an optical fiber by determining the PSPs of the optical fiber and delaying one PSP with respect to the other.
In one aspect, the invention features an optical compensating apparatus for reducing PMD in an optical signal transmitted through an optical medium. The apparatus includes a polarization controller configured to receive an optical signal propagating through the optical medium, to determine the principal states of polarization of the optical medium, and to transform the optical signal based on the principal states of polarization. The apparatus also includes a delay controller arranged in an optical path of the medium after the polarization controller and configured to receive the transformed optical signal, to generate a signal proportional to the PMD time delay, and to minimize PMD time delay of the transformed optical signal.
Embodiments of this aspect may include one or more of the following features. The polarization controller includes a polarimeter. The delay controller includes a polarimeter. The polarization controller further includes a polarization transformer arranged in the path of the optical signal after the polarimeter. The delay controller further includes a delay transformer arranged in the path of the optical signal after the polarimeter of the polarization controller and before the polarimeter of the delay controller. The polarization transformer includes a quarter-waveplate and a half-waveplate.
In another aspect, the invention features an optical compensating apparatus for reducing PMD in an optical signal transmitted through an optical medium. The apparatus includes a polarization module configured to receive an optical signal propagating through the optical medium, determine the optical properties of the optical medium, and generate a signal for transforming the polarization of the optical signal; a polarization transformer arranged in an optical path of the medium after the polarization module and configured to transform the optical signal based on the signal received from the polarization module; and a delay controller arranged in an optical path of the medium after the polarization transformer and configured to receive the transformed optical signal, to generate a signal proportional to the PMD time delay, and to minimize PMD time delay of the transformed optical signal.
Embodiments of this aspect may include one or more of the following features. The polarization module includes a polarimeter. The delay controller includes a polarimeter. The delay controller further includes a delay transformer arranged in the path of the optical signal before the polarimeter of the delay controller and after the polarization transformer. The polarization transformer includes a quarter-waveplate and a half-waveplate.
In another aspect the invention features a method of reducing PMD of an optical signal propagating in an optical medium. The method includes determining a first principal state of polarization of the optical medium with a polarization controller, and transforming the polarization of the optical signal with a polarization transforming device based on the polarization of the first principal state of polarization.
Embodiments of this aspect can include one or more of the following features. The method further includes determining the time delay between the first principal state of polarization and a second principal state of polarization. The method further includes delaying the first principal state of polarization with respect to a second principal state of polarization. The polarization controller includes a polarimeter. The first principal state of polarization is transformed into a linearly polarized state. The first principal state of polarization is transformed with a quarter-waveplate and a half-waveplate.
The invention has various advantages including, but not limited to, one or more of the following. The apparatus for compensating PMD operates in real time and does not require a fast detector.